Conformational studies of 3-aminomethylene-2,4-pentanedione using vibrational and NMR spectra, and ab initio calculations

The IR, Raman and NMR spectra of 3-aminomethylene-2,4-pentanedione (AMP) H2N-CH=C(COCH3)(2) were measured. According to the NMR spectra in chloroform and more polar DMSO at room temperature, the sample exists as single entity. On the other hand vibrational spectra revealed that in less polar solutions AMP exists as two conformers with EZ or ZZ orientation of acetyl groups whereas in more polar solvent only one EZ conformer is observed. Such interpretation was confirmed also by the temperature-dependent measurements of IR spectra in chloroform. The observed IR and Raman bands were compared with harmonic vibrational frequencies, calculated using ab initio MP2 and B3LYP density functional methods in 6-31G** basis set, and assigned on the basis of potential energy distribution. In addition, the geometries and relative energies of possible conformers of AMP were also evaluated at the same levels of theory and compared with the data from X-ray analysis which revealed that AMP exists in solid state as EZ conformer. The influence of environment polarity on this conformational equilibrium is discussed with respect to the SCRF solvent effect calculations using PCM, IPCM and ONSAGER models. (c) 2007 Elsevier B.V. All rights reserved.